Such retention of the focal position is also often called a retaining focus control since the focal and/or imaging position is maintained for a long period of time. Said period of time can last for several hours or even days.
Such periods of time are, for example, necessary for experiments with live cells, when regulating the incubation or the ambient temperature is often critical and deviations from the focal position (i.e., drifts in the observational direction of the imaging optics) would disrupt or make impossible long-term time series experiments. Also, during microscopic applications, time series are frequently recorded, whereby a focus drift would naturally influence the experiment negatively.
Until now, if at all, the retaining focus adjustment is performed continuously in order to ensure retention of the desired focal position. However, disadvantageously this leads to great stress for the electromechanical components of the motion unit since many small movements are carried out during the retaining focus control.